1. Field of the Invention
The present invention relates generally to allocating antennas in a communication system, and more specifically, to a method and apparatus for selecting and allocating antennas efficiently in a cellular mobile communication system based on a Distributed Antenna System (DAS).
2. Description of the Related Art
Mobile communication systems have evolved into a high-speed, high-quality wireless packet data communication systems that provide data and multimedia services in addition to the voice-oriented services provided through early mobile communication systems. Recently, various mobile communication standards, such as High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA), Long Term Evolution (LTE), and LTE-Advanced (LTE-A) defined in 3rd Generation Partnership Project (3GPP), High Rate Packet Data (HRPD) defined in 3rd Generation Partnership Project-2 (3GPP2), and 802.16 defined in IEEE, have been developed to support such high-speed, high-quality wireless packet data communication services. In particular, LTE has been is a technology capable of facilitating such high speed packet data transmission and maximizing the throughput of the radio communication system with various radio access technologies. LTE-Advanced (LTE-A) is an evolved version of LTE that improves the data transmission capabilities of LTE.
Existing 3rd generation wireless packet data communication systems, such as High-Speed Downlink Packet Access (HSDPA), High-Speed Uplink Packet Access (HSUPA) and High-Rate Packet Data (HRPD) systems, use technologies such as Adaptive Modulation and Coding (AMC) and Channel-Sensitive Scheduling to improve transmission efficiency. Through the use of AMC, a transmitter can adjust an amount of transmission data according to a channel state. When the channel state is below a certain quality level (i.e., a ‘Poor’ channel state), the transmitter reduces the amount of transmission data to adjust the reception error probability to a desired level, and when the channel state is at or above a certain quality level (i.e., a “Good” channel state), the transmitter increases the amount of transmission data to adjust the reception error probability to the desired level, thereby efficiently transmitting a large volume of information. With the use of the Channel-Sensitive Scheduling-based resource management method, the transmitter selectively provides services to a user having a better channel state from amongst several users, thus increasing the system capacity, in contrast to methods that include allocating a channel to one user and servicing the user with the allocated channel. This capacity increase is referred to as multi-user diversity gain. The AMC technique and the Channel-Sensitive Scheduling methods each include applying an appropriate modulation and coding scheme at a most-efficient time determined according to partial channel state information fed back from a receiver.
In conjunction with a Multiple Input and Multiple Output (MIMO) scheme, the AMC technique can be used to determine a number of spatial layers for transmission or rank. When using the AMC technique in this manner, the AMC scheme is implemented in consideration of the number of layers to be used in MIMO transmission as well as a coding rate and modulation level.
Meanwhile, research is being conducted in order to find ways to replace the Code Division Multiple Access (CDMA) as the multiple access scheme of the 2nd and 3rd generation mobile communication systems for Orthogonal Frequency Division Multiple Access (OFDMA) in next generation systems. 3GPP and 3GPP2 have started standardization of evolved systems using OFDMA. OFDMA utilizes a larger system capacity than a system capacity utilized through CDMA. One of the significant factors contributing the increase of system capacity of OFDMA relative to CDMA is the use of frequency domain scheduling. Similar to the channel sensitive scheduling based on the time-varying characteristic of channels, it is possible to obtain more capacity gain by using the frequency-varying characteristic of the channels.
In conventional technologies, the cellular system is configured with a plurality of cells as shown in FIG. 1 in order to provide mobile communication with the aforementioned techniques.
FIG. 1 is a schematic diagram illustrating a cellular system including three cells each centered around an antenna.
Referring to FIG. 1, a cellular system includes three cells 100, 110, and 120, and reference numeral 160 denotes an exemplary configuration of the cell 100. The cell 100 is centered around the antenna 130 and serves User Equipments (UEs) 140 and 150 in its coverage area. The antenna 130 provides the UEs 140 and 150 located in the cell 100 with a mobile communication service. The UE 140 is located further away from the antenna 130 than the UE 150, such that the UE 140 is served by the antenna 130 at a lower data rate than the UE 130.
As shown in FIG. 1, each cell is configured in the form of a Central Antenna (CAS) antenna system in which the cell is centered around the antenna. In CAS, although multiple antennas are allocated to each cell, the antennas are arranged at the center of the cell to serve the UEs in the service area. In case that antennas in each cell of a cellular mobile communication system are arranged and managed in the form of CAS as shown in FIG. 1, it is necessary to transmit reference signals for measuring downlink channel condition for each cell. In a 3GPP LTE-A system, a UE measures the channel status between the UE and an evolved Node B (eNB) using a Channel Status Information Reference Signal (CSI-RS) transmitted by the eNB.
FIG. 2 is a diagram illustrating a configuration of a resource block including CSI-RSs transmitted by the eNB.
Referring to FIG. 2, reference numerals 200 to 219 denote paired positions paired for signals of two CSI-RS antenna ports. For example, the eNB transmits the downlink estimation signals for two CSI-RS antenna ports at the position 200. When the cellular system includes of a plurality of cells, such as in the example shown in FIG. 2, the CSI-RS can be transmitted at the positions allocated for each cell. For example, the cellular system can be configured such that the cell 100 of FIG. 1 transmits CSI-RS at positions 200 of FIG. 2, while the cell 110 transmits CSI-RS at positions 205, and the cell 120 transmits CSI-RS at positions 210.
The different time-frequency resources are allocated for CSI-RS transmission of different cells in order to prevent the CSI-RSs of the different cells from interfering with each other.
When using the CAS method as shown in FIG. 1, the transmit/receive antennas of each eNB are concentrated at the center of the cell such that, there are limited capabilities for serving UEs located at the cell edge at a high date rate. Therefore, the data rate for providing the communication service to the UE within the CAS-based cell is determined significantly according to the location of the UE. In this respect, in conventional cellular mobile communication systems operating with calls centered around antennas, UEs located at cell edges cannot be effectively served. Meanwhile, in such conventional cellular mobile communication systems, UEs located near the center of cells can communicate at a high data rate.